basic/fileio: use strdup_to_full() in read_stripped_line()

basic/fileio: use strdup_to_full() in read_stripped_line()

The return value of read_stripped_line() is changed. Before we'd return the
number of characters read, but that number was not meaningful after we called
strstrip(). So just return 0 if nothing was read (EOF), and 1 if something was
read (not EOF). All the callers were only checking for <0 or ==0.